Crystallisation temperature

Li and co-workers [63] studied the crystallisation and melting behaviour of poly(]3-hydroxybutyrate (P-HB)-co-P-hydroxyvalerate (P-HV)) and a blend of poly(P-HB-co-P-HV)/polypropylene carbonate (30/70 w/w) using DSC and FT-IR spectroscopy. Transesterification occurred between poly(P-HB-co-P-HV) and polypropylene carbonate during the melt blending process. During crystallisation from the melt, the crystallisation temperature of the blend decreased by 8 °C compared with that of neat poly(P-HB-co-P-HV) and the melting temperature decreased by 4 °C. This indicated that the presence of polypropylene carbonate reduced the perfection of the poly(P-HB-co-P-HV) crystals, inhibited by the crystallisation of poly(p-HB-co-P-HV) and weakened its crystallisation ability. The equilibrium melting temperatures of 

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Fig. 7. Dsc scan for glassy Alg Ni Fe2Gdg alloy where is the crystallisation temperature and T the eutectic temperature.

Although 16 different crystalline modifications have been identified (24,25), the a-pentahydrate is the stable form below 48°C. Solutions of sodium thiosulfate in the absence of seed crystals can be easily supercooled below their normal crystallisation temperatures. The dotted line extension of the dihydrate phase in Figure 1 is an indication that, if supercooling takes place below this line, solutions normally giving the pentahydrate may form the dihydrate [36989-90-9] s1ste2id. 

The oxidation behaviour of amorphous alloys studied below their crystallisation temperature is not greatly different from that of crystalline metals, although the presence of large amounts of metalloids complicates the situation . ... 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Physical properties to be controlled by these molecular factors include melting point, Tm (only the crystalline part shows a melting point) crystallisation temperature, glass transition temperature, Tg strength,... 

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

EFFECT OF PHYSICAL BLOWING AGENTS ON CRYSTALLISATION TEMPERATURE OF POLYMER MELTS... 

HDPE and LDPE) and on the rate at which the polymer has crystallised (so on the crystallisation temperature). 

Fig. 56. The coercive field of different Terfenol-D/Fe multilayers with Terfenol layer thickness rs,r c = 10 nm, and Fe layer thickness tMf, — 2 nm ( ), 6 nm (TSB) and 8 nm (A). The dashed line shows the crystallisation temperature of a 2-fim thick Terfenol-D film. After Father and Kronmiiller (2000a, 2000b).

Polyethylene crystals change shape with time at temperatures between the crystallisation temperature and the final melting point [42]. The process is commonly referred to as crystal thickening because the major effect is that the crystals increase their dimensions along the c axis. These changes may occur without any change in crystal volume, i.e. crystal thickening is associated with a lateral contraction of the crystals (Fig. 9). A second option is that... 

It is important to point out that several different superstructures are formed in polyethylene depending on the molar mass of the polymer and the crystallisation temperature (Fig. 16). Linear polyethylenes (narrow molar mass distribution Mw/Mn 1.1) of molar mass between 18,000 and... 

Crystallisation of most polymers is accompanied by the separation of different molecular species, a process referred to as molecular fractionation. Bank and Krimm [147] provided the first direct evidence of molecular fractionation in polyethylene. The first extensive study performed by Wunderlich and Mehta [148] indicated that, at each crystallisation temperature, there exists a critical molar mass (MCIjt) such that the molecules of molar mass greater than Mcrjt, are able to crystallise at this temperature, whereas... 

Fig. 26 Critical molar mass of melt-crystallised linear polyethylene as a function of crystallisation temperature. Filled circles data for a broad molar mass sample with Mn=8500 g mor1, Mw=153,000 g mor1 of Mehta and Wunderlich [149], Open circles data for a sample with Mn=12,900 g mor1, Mw=108,000 g mol-1 from Gedde et al. [152]. From [91] with permission from Kluwer, Doordrecht, Netherlands...

At high crystallisation temperatures, the high molar mass polymer crystallised alone. Data for the fold surface free energy obtained from linear growth rate data supported the view that the nature of the fold surface of the dominant lamellae was related only to the molar mass of the crystallising component and was not affected by the composition of the melt. 

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